The invention relates to a gas laser and a dedusting unit thereof for removing dust particles generated during normal laser operation from a gas mixture within the gas laser containing a laser gas and a buffer gas.
Gas lasers are generally known in the art wherein a laser gas is excited by an electrical power source to generate a longitudinal discharge of light. In a typical structure, the laser gas is contained within a tube having a laser discharge zone defined therein between a pair of laser electrodes. The laser gas is excited by applying a relatively high voltage, resulting in the generation of light that can be directed by appropriate optic elements to provide a laser beam. The laser is normally operated or fired in a pulsed manner by connecting a main electrode circuit to the electrical power source at a selected operating frequency. A circulation fan is commonly provided within the laser tube for circulation of the gas mixture through the laser discharge zone.
During laser operation, dust particles are generated within the tube, particularly in an excimer laser. These dust particles are produced by a combination of erosion of metal and insulator material components such as the electrodes in the laser discharge zone, and chemical corrosion of said components by the aggressive laser gas. Dust within the gas has to be avoided since it diffuses the light resonating in the gas discharge gap between the electrodes, thereby lowering the laser light output power significantly. Moreover, the dust particles can accumulate on the optic elements, resulting in potentially significant reduction in the power of the light beam discharged from the tube.
A variety of dust particle filtration systems have been proposed in an effort to reduce the impact of dust particle generation on laser operation. From DE-C-32 12 928 a gas laser is known, in which an external electrical dedusting device is mounted to remove dust particles from the laser gas. Such systems, however, are relatively complex with respect to the assembly of structural components and related circulation of the laser gas to and from the laser housing. U.S. Pat. No. 5,319,663 describes a gas laser including an electrostatic dedusting device directly mounted within the laser housing for flow-through gas circulation during laser operation. However, these devices have not been highly efficient in operation, typically collecting only a small proportion of the generated dust entrained within the gas flow stream during each pass therethrough. Moreover, since the whole gas flow circulating within the tube has to pass through this electrostatic dedusting unit, undesired turbulences are generated within the gas flow. Further, U.S. Pat. No. 5,729,564 discloses a gas discharge laser within a housing that accommodates an electrostatic precipitator for flow-through passage of the laser gas circulated by a fan. However, since the entire flow passes the precipitator, turbulences are generated within the gas flow during the laser operation. Additionally, the precipitator has a large sized structure and, therefore, requires too much space within the housing.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
An object of the present invention is to provide a gas laser and a dedusting unit thereof wherein a continuous circulation of the gas flow within the gas laser is achieved while the gas laser has a reduced, very compact size.
According to the present invention, the gas laser comprises a tube containing a gas mixture including a laser gas and possibly a buffer gas. The tube preferably comprises a cylindrical inner wall. An elongated high voltage electrode is disposed within the tube and extends parallel to the longitudinal axis of the tube. An elongated ground electrode is also disposed within the tube and extends parallel to the high voltage electrode. The ground electrode is spaced apart from the high voltage electrode to thereby define a gas discharge gap therebetween. A circulation means, such as a fan, is positioned within the tube for generating a gas flow within the tube that passes between the electrodes. And, a dedusting unit is mounted along the inner cylindrical wall of the tube in such a manner that a bypass flow which is a part of the gas flow within the tube passes through the dedusting unit.
The elongated high voltage electrode and ground electrode are preferably mounted on the same electrode plate to thereby form a modular discharge unit. The discharge unit may then be removably inserted into the tube.
By passing only a portion of the gas flow within the laser tube through the dedusting unit, turbulences within the circulating gas are avoided, resulting in a continuous gas circulation within the tube during the laser operation. Therefore, the electrical gas discharge gap between the two electrodes is provided with a continuous gas flow to achieve an effective ionization of the laser gas.
Additionally, since the dedusting unit is mounted along the inner cylindrical wall of the tube, the structural size of the gas laser may be reduced over prior art designs. In a particular preferred embodiment of the present invention, the dedusting unit is directly mounted to and integrated within the tube to provide a compact and operationally efficient gas laser. The centrifugal force of the bypass gas flow towards the inner cylindrical tube wall adjacent to the dedusting unit, helps to effectively dispose the dust particles at this cylindrical wall portion, without deflecting this flow in an undesired direction. Because the dedusting unit charges the dust particles electrostatically, they remain adhered to the cylindrical wall portion of the tube.
Preferably, the inner cylindrical wall of the tube is circular in cross section. This allows the gas flow to efficiently circulate within the tube during the laser operation at a uniform velocity of gas flow.
In a preferred embodiment of the invention, the dedusting unit comprises a partition wall that has a substantially circular middle section extending substantially parallel to the inner cylindrical wall. The arrangement and design of the partition wall enables the gas flow to exactly control the bypass flow between the partition wall and the inner cylindrical wall for dedusting the bypass flow. For effective dedusting, it is sufficient if only a minor part of the whole gas flow is directed through the dedusting unit.
The bypass flow is directed through a mouth portion having a substantially constant cross section and a defined orientation with respect to the partition wall and the inner cylindrical wall, so that an efficient and uniform dedusting of the bypass flow is secured. The remaining, or main, gas flow is guided along the side of the partition wall facing away from the inner cylindrical wall. This main gas flow is continuously supplied together with the joining dedusted bypass flow to the circulation means. The circulation means again circulates the gas through the discharge gap and then through the dedusting unit where it is once again divided into a mainflow and bypass flow for dedusting the same.
The dedusting unit preferably includes a U-shaped channel extending parallel to the electrodes and having perforated walls that allow the bypass flow to penetrate the wall of the U-shaped channel. The U-shaped channel may be compactly formed and can be easily mounted to the dedusting unit. The perforated walls of the U-shaped channel enhance the electrostatic field charging of the dust particles electrostatically for adhering to the inner tube wall.
To form an inhomogeneous electrical field within the dedusting unit for electrostatically charging the dust particles, a high voltage wire may extend lengthwise between the two adjacent walls of the U-shaped channel and be supplied with a high positive voltage. Therefore, the dust particles of the bypass flow are electrostatically charged and attracted by the grounded inner cylindrical wall of the tube so that the bypass flow is reliably filtered within the dedusting unit.
For improved efficiency of dedusting the bypass flow, a plurality of U-shaped channels may extend along the length of the tube in a parallel, spaced-apart relationship with each other. Further, a high voltage wire may be disposed between the walls of one or more of the U-shaped channels. Therefore, the dust particles are electrostatically charged at several locations within the dedusting unit and captured along the whole inner cylindrical wall in the region of the dedusting unit. This results in an improved efficiency of the dedusting unit.
In a preferred embodiment of the invention, the electrical gas discharge unit extends along one side of the cylindrical wall while the dedusting unit extends along the opposite side of the cylindrical inner wall. By this arrangement of the gas discharge unit in relation to the dedusting unit, a suitable dedusting effect is achieved, because the timing and the pressure proportions of the gas flow during the laser operation are controlled in such a manner that a sufficiently clear gas flow is supplied to the discharge unit. Additionally, the space within the tube of the gas laser is functionally used for assembling the components of the laser.
Preferably, the circulating means is a radial fan extending along the portion of the cylindrical wall that extends between the discharge unit and the dedusting unit. Therefore, the circulation of the gas flow is optimized and the gas laser can be constructed in such a way that its size is reduced.
In a preferred embodiment of the invention, the outer contour of the discharge unit has a circular section accommodating a section of the radial fan in such a manner that the exhaust side of the radial fan is disposed close to the electrical discharge gap. This allows a sufficient amount of gas flow to be supplied to the electrical discharge gap between the electrodes of the discharge unit, which gas flow has been efficiently cleaned of dust particles in the dedusting unit.
It is preferred, if the gas laser is an excimer laser. Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as for high-resolution lithography. Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example ArF excimer laser) or a gas containing a halide (for example F2) as laser gas. To increase the homogeneity of the gas discharge, in excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be reused, excimer lasers are generally operated in a pulsed operation mode, wherein the laser gas in the discharge gap is being steadily replaced by the circulation means.